As well known to those skilled in the art, in a power transmission device used in a typical forklift truck, a transmission is provided along with a clutch between a torque converter and an axle shaft. Therefore, conventional power transmissions are problematic in that an engine and the axle shaft become excessively long, and most of the vibration and noise from the engine is undesirably transferred through the transmission to a driver s seat provided in a central portion of a motor vehicle.
Furthermore, in the conventional power transmission devices, a transmission is provided between the engine and a bevel gear unit, which changes the direction of power transmission and conducts gear reduction. Accordingly, excessive shock may occur when shifting the gear, thereby resulting in deterioration of durability of the transmission.
In an effort to overcome the problems experienced with the conventional power transmission devices, the present inventor has proposed a compact transmission (Korean Patent No. 10-0442475 (Korean Patent Application No. 10-2004-02909)), which is provided between a bevel gear unit and a differential gear unit. In this compact transmission, a clutch unit is separated from a wheel reduction gear assembly and is provided between a bevel gear and a differential gear so that it is unnecessary for the bevel gear unit to change its rotational direction in order to change speed. That is, because the bevel gear unit rotates in one direction, noise and vibration are markedly decreased, and the number of elements constituting the transmission is reduced. Thereby, the weight and manufacturing costs are reduced.
The compact transmission of Korean Patent No. 10-0442475 will now be described in detail with reference to FIG. 1. As shown in FIG. 1, power generated from the engine of a vehicle is perpendicularly transmitted through a bevel gear unit having both a bevel pinion gear and a bevel ring gear 2. The power is transmitted from the bevel ring gear 2 to a clutch unit which includes a forward clutch part 3 to be connected for forward driving of the vehicle, and a backward clutch part 4 to be connected for backward driving of the vehicle.
The power is transmitted from the clutch to a differential gear casing 10 through the forward clutch part 3 or the backward clutch part 4. The differential gear casing 10 has a differential pinion gear 11 and a differential side gear 12. The differential gear casing 10 is coupled to a main drive shaft 13 and transmits the power to the main drive shaft 13. The main drive shaft 13 is coupled to a reduction gear assembly 14 and transmits the power to the reduction gear assembly 14. Thus, the vehicle moves forwards or backwards.
Each of the forward and backward clutch parts 3 and 4 includes a clutch drum 7, a cylinder which is provided in the clutch drum 7, and a piston 5 which is provided at a predetermined position in the cylinder to be movable by hydraulic pressure of hydraulic oil. Each of the forward and backward clutch parts 3 and 4 further includes a spring 6 which supports the piston 5, and a clutch pack 8 which is coupled to both the inner peripheral surface of the clutch drum 7 and the outer peripheral surface of a coupling 9.
However, in the case of a heavy vehicle with the conventional compact transmission having the above-mentioned construction, when the heavy vehicle is momentarily stopped, using a brake, on an upward slope or when a subsequent operation is conducted in the stopped state on the upward slope, the hydraulic pressure, which has been applied to the forward clutch part, is interrupted because of limited engine output, and the above purpose must be achieved using additional power supplied from the engine.
Furthermore, to restart the vehicle from the stopped state on the upward slope, hydraulic oil must be supplied into the forward clutch part 3 to move the piston 5 in a predetermined direction, thus compressing the clutch pack 8 so that power is transmitted from the engine to the main drive shaft 13.
However, until the pressure of the hydraulic oil supplied into the clutch unit becomes sufficient to transmit power from the engine to the main drive shaft 13, the heavy vehicle moves backwards on the upward slope due to the gravity. Furthermore, to move the backward-moving heavy vehicle forwards on the upward slope, high engine output is required. Therefore, the conventional compact transmission is problematic in that fuel consumption is excessively increased, and because great load is applied to the elements constituting the transmission, the lifetime of the transmission is reduced.
In order to solve the aforementioned problems, the present inventor has developed a compact transmission disclosed in Korean Patent No. 10-0687300 (Korean Patent Application No. 10-2005-0015918).
Referring to FIG. 2, the conventional compact transmission including a clutch unit 200 for controlling power transmitted from a bevel gear unit 100 coupled to an output shaft of an engine, the clutch unit having a forward clutch part 20 and a backward clutch part 40. The forward clutch part 20 and the backward clutch part 40 are coupled to a forward drive gear 26 and a backward drive gear 46 through a forward coupling 25 and a backward coupling 45, respectively, and both coupled to a differential gear part 60, so that power is transmitted from the differential gear part through a main drive shaft 16 and an axle assembly to a reduction gear assembly.
The clutch unit 200 further includes a subsidiary forward clutch part 30. The backward coupling 45 extends in a subsidiary clutch drum 31 of the subsidiary forward clutch part 30 and one or more friction plates are coupled to an inner peripheral surface of the backward coupling 45.
A one-way bearing 36 is fitted on an outer peripheral surface of the forward coupling 25 extending in the subsidiary clutch drum 31, and one or more friction disks 33 fitted on an outer peripheral surface of an outer ring 35 of the one-way bearing 33. A piston 34 supported by an elastic means in the subsidiary clutch drum 31 is moved by hydraulic pressure to compress the friction plates 32 and the friction disks 33. Further, a bearing 37 is inserted between the friction plates 32 and the piston 34.
However, in the conventional compact transmission, since the bearing is disposed between the friction plates and the piston, it may be readily broken by the weights of the friction plate, the piston and the like. Accordingly, there is a demand for a transmission including a subsidiary forward clutch part with a simpler structure wherein the bearing between the friction plates and the piston would not be broken.